Conventionally, nuclear reactors comprise pressure tubes or pressure vessels, though nuclear reactors that have both pressure tubes and a pressure vessel have been disclosed.
At present, nuclear power plant safety is of particular concern in the aftermath of the Fukishama accident in Japan in 2011 and others in which the fuel becomes exposed after the reactor has shut down. Some proposals for improving safety have focused upon prolonging the time to catastrophic failure in a severe accident in which decay heat may cause either or both of fuel cladding failure and hydrogen production. One option is to provide passive decay heat removal following a severe accident but this requires the fuel cladding to be capable of retaining the fuel fission products while the decay heat is transferred from the fuel, predominantly by thermal radiation.
Although improving safety is of paramount importance, it is desirable to do so without reducing efficiency. In fact, there is an ongoing desire to improve the efficiency of nuclear reactor power plants without prejudice to their safety. Improved efficiency can be obtained by increasing operating temperatures. To this end, it has been proposed to build nuclear power reactors employing supercritical water which requires much higher operating pressures than the current art PWR, BWR and PHWR. A fuel must be capable of operating at the temperature and pressure of supercritical water, and withstanding the corrosive environment of irradiated supercritical water and radiation damage. The fuel cladding must also have acceptably low neutron absorption to function economically while desirably providing for passive decay heat removal following a severe accident in which the fuel becomes exposed after the reactor has shut down.
In this industry, a variety of terms are used for the pressure barrier between the fuel and the reactor coolant. For convenience, in the context of this specification, the term “cladding” will be used for such pressure barrier, whether in a pressure-tube or pressure-vessel type of reactor.
Also, the term “fuel elements” will be used to embrace both the fuel elements of a pressure-tube type of reactor and the fuel rods of a pressure-vessel type of reactor.
The term “fuel assembly” refers to a plurality of fuel elements which are held together in parallel. In the case of a PHWR, this fuel assembly usually is called a “fuel bundle”.
The term “fuel channel” refers to an assembly of components in a pressure tube type reactor comprising the pressure tube and other components that maintain and provide insulation between the reactor coolant and moderator outside the pressure tube.
It is known to use supercritical water systems in fossil fuel power stations. However, the technologies, particularly materials, used in supercritical fossil fuel stations cannot necessarily be used in supercritical nuclear reactor stations where low neutron absorption and corrosion resistance at supercritical temperatures and radiation levels are particularly important. This is especially so for the fuels and the fuel assemblies containing them.
It is known, for example, to use stainless steel to clad fuel for a higher temperature operation. It is unlikely that, in severe accident conditions, this fuel cladding would have been capable of retaining fission products while passively transferring decay heat in a PHWR, PWR or BWR. In particular, under severe accident conditions, the temperature will become high enough to cause the conventional fuel cladding to oxidize and eventually melt, leading to hydrogen production and release of fission products.
Other steels, nickel and titanium-based alloys that have been studied for supercritical water reactor use also have relatively high neutron absorption and entail the use of enriched uranium. They would not be entirely suitable for use in applying similar reactor physics when refitting an existing reactor, for example a PHWR.
In his concurrently-filed patent application Ser. No. 13/829,812, the present inventor discloses fuel elements in which the fuel pellets are housed in a cladding tube made of sapphire. It is known to make sapphire tubes by growing the sapphire using edge defined film fed growth and using a die to form it into a tube. Although sapphire tubes made in this way may be satisfactory for general application, they would be of limited use as cladding in nuclear reactor fuel elements because their inner surfaces, as initially manufactured, have ridges that limit thermal contact between the fuel pellets and the cladding tube.
While it might be possible to grind the inner surfaces of the sapphire tubes to obtain a required smoothness for cladding, grinding would require very hard, for example diamond, grinding tools and be very time-consuming to obtain required cladding and component dimensions. Consequently, it may not be economical and commercially viable at this time.